Impedance spectroscopy: Over 35 years of electrochemical sensor optimization

Abstract

There is considerable interest in the development of electroanalytical sensors (i.e., potentiometric, amperometric, electrochemical biosensors) for the detection of a wide range of analytes. The success of many of these sensors is governed by the condition and stability of the membrane/electrode surface. In fact, the response mechanism is dictated primarily by the surface structure and a considerable amount of work has been undertaken to characterize the interfacial region. Consequently, electrochemical impedance spectroscopy (EIS) has played a pivotal role in the characterization of many types of sensors. EIS has been used to provide information on various fundamental processes (i.e., adsorption/film formation, rate of charge transfer, ion exchange, diffusion, etc.) that occur at the electrode–electrolyte interface. Understanding and manipulating these interfacial processes has assisted in the development of membranes/electrodes with new and improved response characteristics. This paper reviews some of the work that has been undertaken using EIS over the past 35 years. More importantly, it evaluates the power of EIS in characterizing a wide range of electrochemical sensor systems.

Introduction

Significant technological advances over the past few decades have led to the development of numerous analytical devices in the monitoring of a wide range of analytes. The desire to measure and understand every single variable in our environment has been the impetus behind the growth of many innovative tools. More specifically, electrochemical-based sensors are one group of analytical devices that have attracted considerable attention in recent times. The introduction of electrochemical sensors in the last century has revolutionized the way in which we lead our lives. This is not surprising considering that they play a crucial role in medical and clinical analysis, environmental and industrial monitoring [1], [2], [3], [4], [5], [6]. The common feature binding all electrochemical sensors is that they rely on the detection of an electrical property (i.e., potential, resistance, current), and are normally classed according to the mode of measurement (i.e., potentiometric, conductometric, amperometric). A number of excellent reviews appear on the topic [7], [8], and the authors recommend them for further reading. Indeed, electrochemical sensors is a large field that continues to evolve, and this is partly due to the fact that the underlying electrochemical principles for the detection of analytes are highly relevant in many diverse areas.

A fundamental challenge pertinent to electrochemical sensor design lies in the molecular understanding of the relationship between surface structure and reactivity. The fabrication of sensor materials with unique response characteristics has created a pressing need to understand their chemical and physical properties. Understanding the fundamental processes that govern sensor response in most cases leads to the development of electroanalytical devices with superior selectivity, excellent chemical stability, higher sensitivity, and lower detection limits. In order to achieve these objectives, it has been necessary to study various electrical processes that occur at the surface of the sensor or inside the sensor membrane itself. This may involve tailoring of the chemistry of the surface layer so that it exclusively adsorbs the target molecule/ion in the presence of interferences, or it could involve altering the bulk electrical conduction properties of the sensor by modifying the membrane composition. Sensor optimization is one of the most crucial steps in the realization of an electroanalytical device. Whether it involves tinkering with the interfacial properties or modifying the membrane composition, a suitable tool that allows the sensor performance to be evaluated under a range of conditions must be employed. Understanding how various parameters influence the response mechanism and interfacial reaction kinetics will assist with the development of electrochemical sensors with new and improved response characteristics.

There are countless techniques available for electrochemical sensor development/optimization (i.e., cyclic voltammetry, scanning electrochemical microscopy, etc.) [9], [10], [11], and it is beyond the scope of this review to discuss the relative merits of each technique. A majority of these methods probe the membrane/electrolyte interface by using a large perturbation, which is designed to provide mechanistic information by driving the reaction to a condition far from equilibrium. Another approach, however, is to apply a small perturbation to ensure that the kinetic information pertaining to the membrane/electrolyte interface is at near zero current conditions. Subsequently, electrochemical impedance spectroscopy (EIS) is a non-destructive steady-state technique that is capable of probing the relaxation phenomena over a range of frequencies [12]. The power of EIS lies in its ability to provide in situ information on relaxation times over the frequency range 10⁶ to 10⁻⁴ Hz. It is a tool that has been used to identify and separate different contributions to the electric and dielectric responses of a material. Traditionally, EIS has provided a wealth of information pertaining to the corrosion rate and corrosion processes on a wide variety of metals and metal-coated surfaces [13], [14]. Nowadays, we are finding an increasing use of EIS to investigate the adsorption and charge transfer processes of many types of electrochemical sensors.

The purpose of this article is to review recent advances and applications of EIS in unravelling the response mechanisms of electrochemical-based sensors. Consequently, the authors will evaluate the effectiveness of EIS as a technique for investigating sensor response processes, and have aimed the article at a specialized electrochemistry readership. Ultimately, the review focuses on the use of EIS in the development of electrochemical-based sensors and biosensing devices.

If a novice to the EIS technique finds themselves interested in the subject matter of this fundamental review then the authors strongly recommend the superb EIS text by Macdonald [12], along with a compilation of excellent research articles [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], to obtain a good grounding in the fundamental principles underpinning the EIS technique, as this rudimentary information is beyond the scope of this specialized review paper.